How to use a value from a drop-down as a filter in a SELECT query?

[StarLavender]

Hi. I want to have a query as below.

$sql = "SELECT amount FROM payment WHERE id = '$a'";
$result = mysql_query($sql);
$row = mysql_fetch_array($result);
$amount = $row['amount'];

The value for $a variable is getting from the selection on drop down list. For example, the user select id=11, then the amount for this id is 10 which select from database. So how am I going to assign the value to $a based on the drop down selection of the user? Thanks in advance for any help.

[code green]

The value is written in the HTML like

<option name="samename" value="10">Choice

Then in php

$a = $_POST['samename'];